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05-31-2014, 07:27 PM   #1
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Confused about APS-C crop factor

OK. I'm trying to wrap my mind around APS-C crop factor and I do understand that it is related to the size of the APS-C sensor in relation to the 35mm film size. I get that a 50mm lens designed for 35mm cameras is equivalent to a 75mm lens on a 35mm film camera (1.5x the focal length). What I don't understand and hope you fine folks can clear up for me is the following ...

Does this only apply to lenses not specifically designed for APS-C digital cameras? For lenses specifically made for digital cameras (like the Tamaron Di or Pentax DA series, etc), does crop factor apply? Is my Pentax DA 35/2.4 seem like a 52.5mm lens on my DSLR or is it truly a 35mm?

Does the crop factor really just tell one what FOV to expect when using a lens. Is it just a relative "zoom" factor based on FOV and the crop. The image is not actually bigger, right? it just a reduced FOV?

And finally -- does my quandry make any sense?? Am I nuts for letting this annoy me to the point that I am losing sleep over it. (J/K) Every time I buy a wide angle lense I keep thinking I now need to buy a lense 1.5x wider in order to get what I just paid for. Help!!

05-31-2014, 07:36 PM   #2
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The crop factor applies regardless of the type of lens. A 50mm film era lens would give you the same field of view as a 50mm DA lens, for instance. And yes, all the crop factor affects is field of view.

So you get the same FOV with a 16mm on apsc as with a 24mm on full frame, and so on

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05-31-2014, 07:41 PM   #3
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The focal length of a lens is a fixed optical property, the crop factor is a short hand way of calculating the difference in the field of view. https://www.google.com.au/search?q=focal+length&biw=1920&bih=969&tbm=isch&im...ml%3B450%3B320

What does that mean? a 50mm lens will have a particular FOV on a 35mm camera (digital or film) and on a APS-C sized sensor will have a FOV that is similar to a 75mm on a 35mm camera.

If a lens is designed for use with APS-C sized sensors then you still need to apply a crop factor if you want to know what the FOV would be compared a 35mm camera, the lens just won't work on a 35mm camera
05-31-2014, 08:45 PM - 1 Like   #4
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APSC crop factor confusion

This is a really contentious area. A long focal length will compress a view (appear to make near and far objects closer to each other) whereas a wide angle will expand a view of a scene (make near and far objects appear further away from each other). This means that a 50mm on a Pentax APSC is not strictly the equivalent of a 75mm lens but the equivalent of a 50mm lens which has been cropped (ie pretend you took the photo with a 50mm lens on a FF camera and then cropped the image). The field of view will be as if you'd used the 75mm lens but the relation of the near and far objects in the photos will be that of a 50mm lens. Confused? We all are.


Last edited by officiousbystander; 05-31-2014 at 08:49 PM. Reason: Added further to the explanation to make it clearer.
05-31-2014, 08:51 PM   #5
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Thanks folks. That's what I thought. So with a 35mm -based lens, the image presented is larger than the sensor and since it falls outside of the sensor it is discarded thus rendering a reduced FOV. As I understand it, then what one is actually capturing is approx. the center of the entire Full Frame sized projected image. So if even APS-C designed camera lenses also need to have the crop factor accounted for, it would seem that there is an ever decreasing return as one goes wider and wider angle lenses. So if this is true, what is the widest angle lens one can realistically expect to get before the increase in FOV on a APS-C is indiscernible?

QuoteOriginally posted by officiousbystander Quote
This is a really contentious area. A long focal length will compress a view (appear to make near and far objects closer to each other) whereas a wide angle will expand a view of a scene (make near and far objects appear further away from each other). This means that a 50mm on a Pentax APSC is not strictly the equivalent of a 75mm lens but the equivalent of a 50mm lens which has been cropped (ie pretend you took the photo with a 50mm lens on a FF camera and then cropped the image). The field of view will be as if you'd used the 75mm lens but the relation of the near and far objects in the photos will be that of a 50mm lens. Confused? We all are.
OK. Now my head REALLY hurts. :/
05-31-2014, 09:02 PM - 1 Like   #6
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Lenses cast a circular view of an image with a particular field of view. We forget this because our films and sensors are rectangular (or square in some cases with film photography). Try this exercise.

Draw a circle.

Now draw a rectangle inside the circle. See how much of the circle got cropped?

Next draw a stick figure inside the rectangle.

Go on and draw a smaller rectangle inside the larger rectangle. Do you see how much more got cropped? You may have cropped the stick figure too.

This is how the APS vs. FF (or larger) sensor cropping works. If your stick figure was cropped after the second rectangle then you would have to zoom out or step back to make the figure smaller to begin with in order for him to fit inside the second rectangle. The original circle didn't change but your view (i.e. the rectangle) got narrower as the rectangle got smaller.

Now imagine taking the two views given by both rectangles and expanding them one at a time to fill your computer screen. Each will give a different field of view even though the focal length of the lens is the same. People always compare focal lengths against the field of view that would be achieved with 35mm film. The field of view achieved by a 50mm on a APS-C sensor is the same as if you had a 75mm lens on 35mm film.

APS lenses differ from FF lenses with their image circle. Lens designers will have the lens project a smaller circle to begin with because they know that the sensor will do some serious cropping otherwise. Why bother designing something that will be cropped? To understand this just draw a smaller circle inside your first circle. Make the second circle encompass the smaller rectangle. Keep your stick figure the same size.

The effect can be verified by mounting an APS lens on a film camera body. For example, try mounting the DA 18-135mm on any Pentax film camera. Zoom out to 18mm. What do you see? Yikes! 50% of the image along the edges is missing! Why? The lens was designed to cover an APS sensor instead of 35mm film.

Disclaimer : Many APS optical formulas are based upon historical designs which were tailored around 35mm film. That means there are APS lenses that would cover 35mm film. However, the performance at the edges is not guaranteed. It may be OK or it may be awful!

Note : Using a FF lens on an APS sensor will create a crop but it also induces a lot of optical performance strain on the lens. Aberrations that were once very small relative to 35mm film now become amplified on an APS sensor. Remember the exercise of taking the areas from the two rectangles and stretching them on your fixed computer screen? I'm taking a smaller area and expanding 1.5x more than the other.
05-31-2014, 09:07 PM - 1 Like   #7
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QuoteOriginally posted by ripper2860 Quote
OK. Now my head REALLY hurts. :/
The best way to avoid the crop factor head-ache is to forget you ever heard the term. You may also wish to forget you ever heard the term "35mm equivalent" as well. Use this cheat-sheet instead for categorizing field of view:
  • Less than 15mm = ultra wide angle
  • 16mm --> 26mm = wide angle
  • 27mm --> 39mm = normal
  • 40mm --> 85mm = short tele/portrait
  • 86mm --> 200mm = tele
  • 200+ = long tele


Steve
05-31-2014, 09:31 PM   #8
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I would second stevebrot's approach.
Trying to figure crop factors is like trying to convert all measurements from imperial to metric before doing any calculations. Better to just forget the old system and work entirely in the new.
That chart is pretty well how I see it though I have a tendency to think of specific Pentax lenses as the cutoffs for those groups rather than fixed numbers.

I still use both systems as I like the old film cameras better, but I find myself thinking in totally different angles when I use my lenses on my crop body vs my film cameras, you just get used to the second system.
Or perhaps you don't get used to it at all and spend most of your time cursing the tiny viewfinders and toilet paper tube field of views, but you learn it anyways.
What I can't get used to is the wide end of even the kit lens making everything in the image so tiny you can't see any detail anymore. Used to be that 15mm would swallow the whole world with its FOV and still show enough detail to enjoy it. I personally dislike dipping below 24mm for this reason.
But now I have gone and digressed into a stick in the mud rant.

05-31-2014, 09:44 PM   #9
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Thanks all and thanks Steve. The "cheat sheet" will certainly help as I look to round out my budget Prime lens stable and serve as a guide to selecting the right lens for particular shooting scenarios.. Now this brings me to another, somewhat related question ...

Why are Prime Wide Angle lenses (21-25mm) so much more expensive? Even more than telephoto or zoom lenses. You would think that the complexity of a zoom or materials of a Prime Tele would cause them to be more expensive -- yet wide angles seem to consistently be higher priced. Is it Supply and Demand or are wider angle lenses inherently more difficult to engineer due to less space and less light?

Again -- thanks everyone!!

Last edited by ripper2860; 06-01-2014 at 10:12 AM.
05-31-2014, 09:48 PM   #10
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QuoteOriginally posted by stevebrot Quote
The best way to avoid the crop factor head-ache is to forget you ever heard the term. You may also wish to forget you ever heard the term "35mm equivalent" as well.
+1 or maybe a whole bunch. Unless you shoot both APS-C and FF digital or film then you can simply forget you ever heard the term 'crop factor'. If all you have ever shot is APS-C cameras then a 35mm lens looks like a 35mm lens and a 50mm lens looks like a 50mm lens, you have no frame of reference that says "this looks different than it did on film".

While I shot film years ago, when I returned to photography it has all been APS-C so if I ever get a FF camera all of my lenses will look weird compared to what I am used to, which is APS-C.
05-31-2014, 09:53 PM   #11
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QuoteOriginally posted by jatrax Quote
If all you have ever shot is APS-C cameras then a 35mm lens looks like a 35mm lens and a 50mm lens looks like a 50mm lens, you have no frame of reference that says "this looks different than it did on film".
I shoot multiple formats from pinhead (phone cam) through to 4x5 film and rely on simply knowing my gear as the key to knowing the proper lens for the subject and composition.


Steve
05-31-2014, 10:07 PM   #12
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QuoteOriginally posted by ripper2860 Quote
Thanks all and thanks Steve. The "cheat sheet" will certainly help as I look to round out my budget Prime lens stable and server as a guide to selecting the right lens for particular shooting scenarios.. Now this brings me to another, somewhat related question ...

Why are Prime Wide Angle lenses (21-25mm) so much more expensive? Even more than telephoto or zoom lenses. You would think that the complexity of a zoom or materials of a Prime Tele would cause them to be more expensive -- yet wide angles seem to consistently be higher priced. Is it Supply and Demand or are wider angle lenses inherently more difficult to engineer due to less space and less light?

Again -- thanks everyone!!
Someone may want to double check me and correct me if I'm wrong here ....

Wide angle lenses tend to have a fisheye effect. This can be noticed by zooming out and then getting real close to someone for a portrait. Their nose will be huge! Most photographers want a flat field across the lens. Flattening the field requires special black magic sorcery and wizards for hire are not cheap. Telephoto lenses tend to "pinch" the image but the effect is much less severe than the fisheye effect at the shorter focal lengths.

The photozone.de website has some good plots of the field distortion created by different lenses. Check out the two extremes.

DA 14mm
Pentax SMC-DA 14mm f/2.8 ED [IF] - Review / Test Report - Analysis

DA* 200mm
Pentax SMC DA* 200mm f/2.8 ED [IF] SDM - Review / Test Report - Analysis

Some of this distortion can be corrected in the camera or during post processing but nothing is free. There will be compromise in resolution. Some pixels will be doubled and stretched while others will be combined and smushed. Don't get too carried away in thinking about distortion. Every lens will distort the image to some degree. As long as you are happy with the resulting image then the number crunching has little value except to create a number.
05-31-2014, 10:23 PM   #13
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QuoteOriginally posted by stevebrot Quote
I shoot multiple formats from pinhead (phone cam) through to 4x5 film and rely on simply knowing my gear as the key to knowing the proper lens for the subject and composition.
I know. I just meant that if the OP is APS-C only then just ignore the crop thingy. If you shoot multiple formats then you need to know your gear and what it will do for you. I shot film for many years, then walked away from photography for almost 20 years. Since returning it has been APS-C only for me and all my eye knows is that format. I'll need to retrain if I ever move to FF.
05-31-2014, 11:11 PM - 1 Like   #14
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QuoteOriginally posted by 6BQ5 Quote
Wide angle lenses tend to have a fisheye effect. This can be noticed by zooming out and then getting real close to someone for a portrait. Their nose will be huge! Most photographers want a flat field across the lens.
The volume anamorphosis (that's a mouthful) of facial features has nothing to do with fisheye lenses* or with flat field optics. What it does have to do with is the rules of perspective which are driven by the position of the lens relative to the subject. Even flat field lenses with excellent rectilinear projection exhibit this feature. What is closer appears larger because it is closer. The only reason the subject appears distorted is that the wide angle lens provides a wider field of view than a "normal" lens at the same distance and allows us to see elements of the subject that are at greater relative distance. You can test this by doing a crop of a "distorted" big nose/chin wide angle portrait and comparing it to a photo taken with a longer lens from the same camera position.

It cannot be stated strongly enough that this is the case for any lens that provides a wide field of view for a particular frame size (format) and is not a property of focal length per se. 28mm is a wide angle focal length on my 35mm film and will provide a very satisfactory duck face when mounted with my KX film camera. The same 28mm lens on my smaller APS-C K-3 provides a fairly flat and boring "normal" perspective.

Steve

* As with all wide angle optics, fisheye lenses will record anamorphosis, but it is unrelated to the lens' optical design which counter-intuitively actually provides less true volumetric distortion of objects at edge of field than a rectilinear lens of the same FOV.

---------- Post added 05-31-14 at 11:32 PM ----------

QuoteOriginally posted by jatrax Quote
I know. I just meant that if the OP is APS-C only then just ignore the crop thingy
Sorry I was not more clear

It was my intent to affirm and agree with what you had said. My bad.


Steve

Last edited by stevebrot; 05-31-2014 at 11:57 PM.
06-01-2014, 02:27 AM   #15
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QuoteOriginally posted by 6BQ5 Quote
Note : Using a FF lens on an APS sensor will create a crop but it also induces a lot of optical performance strain on the lens. Aberrations that were once very small relative to 35mm film now become amplified on an APS sensor.
Ok, this I don't buy.

Lens abberations don't change just because the capture area in the focal plane is smaller. If anything, abberations "go away" because most are near the outer edges of the projection circle, which is no longer near the sensor.
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